Phong Nha
Phong Nha-Ke Bang National Park, in central Vietnam’s Quang Binh province, holds the largest cave system on Earth. Phong Nha Cave itself is a boat ride into a limestone mouth on an underground river; Paradise Cave is a kilometres-long walk past pillars and curtains of stone; and Son Doong, only opened to limited expeditions in 2013, is large enough to contain a jungle and its own weather. The surrounding karst country is jungle-thick and largely unpaved — the town of Phong Nha is the obvious base, reachable by overnight train from Hanoi or Hue.